1. Field of the Invention
The present invention relates to a nuclear magnetic resonance (NMR) machine with low field and dynamic polarization. The NMR machines concerned are more specifically NMR imaging machines which find application in particular in the medical field. The object of the machine in accordance with the invention is to improve the signal-to-noise ratio of the detected signal in order to achieve enhanced sharpness of detail of the images obtained. Moreover, the invention can make a significant contribution to reduction in cost of such machines by simplification of their homogeneity correction coils.
2. Description of the Prior Art
Classically, an NMR machine essentially comprises a magnet, or a coil which performs the same function, in order to subject a body to be examined to an intense and permanent orienting magnetic field. When subjected to this influence, the body is then excited electromagnetically by a high-frequency electromagnetic wave. On completion of the excitation, a measurement is performed on a de-excitation electromagnetic wave which is emitted by the body and provides information on the intimate nature of said body. It is known that the amplitude of the detectable electromagnetic signal in such machines is of the type .chi. B.sub.O.sup.2. In this expression, .chi. is the magnetic susceptibility of the body to be examined and B.sub.O is the intensity of the orienting field of the machine.
Now the amplitude of the noise re-emitted by a body is proportional to .omega., where .omega. is the resonant angular frequency of the magnetic moments of the body particles when the body is subjected to the influence of the orienting field. The immediate result thereby achieved is that the signal-to-noise ratio of the detectable electromagnetic wave is proportional to B.sub.O since .omega. is itself proportional to B.sub.O. In the present state of the art, this observation has favored the construction of NMR machines with an orienting field B.sub.O which is as large as possible. This incentive is greater by virtue of the fact that, with a high field, the resistance provided by the antenna in parallel with the resistance offered by the body becomes negligible.
In a high-field machine, the magnetization given to the particles of a body to be scanned is substantial. This magnetization is related to the polarizing force of said field and is stronger as the field is higher. Furthermore, at the moment of excitation, the spins of the particles begin to resonate at a frequency which is also proportional to the orienting field B.sub.O. At the moment of excitation, the orienting field therefore performs in addition to its polarizing field function, a second function which is that of a resonance field. Just as the use of a high-intensity orienting field is useful in regard to polarization and magnetization since it increases the amplitude of the detectable signal, so the use of a high field at the moment of resonance is a difficult matter.
In fact, if the orienting field is not perfectly homogeneous, two types of disadvantages accordingly arise. Minor consequences first appear in regard to magnetization (on condition that inhomogeneity is nevertheless not excessive). In fact, this different magnetization at different locations of the machine produces a defect in homogeneity of luminosity of the image which can be arranged. But other more critical consequences affect the detectable electromagnetic signal. In fact, if the resonance field is of the order of 1 Tesla and if the particles examined are hydrogen particles (present in water and therefore in a large quantity in human bodies), the resonance frequency of the spins is of the order of 40 MHz. An inhomogeneity of one millionth of the value of the resonance field therefore produces a resonance deviation of approximately 40 Hz. This means that, at the end of a period of 12.5 ms, contributions to the total NMR signal made by adjacent particles but subjected to resonance fields which differ from each other by one millionth are in phase opposition. The immediate result is that the total electromagnetic signal (the only one which is detectable) is then cancelled.
Phase dispersion of the different contributions to the NMR signal by the signals emitted by the different particles can be countered by the use of two techniques. Firstly, it is possible to adopt a so-called spin echo technique. In this latter, with an additional electromagnetic excitation pulse, one produces after application of the excitation pulse a reflection of the phase dispersion so that the NMR signal reappears at the end of a time interval of double the value of the time interval which elapses between the excitation pulse and the reflection pulse (or so-called echo pulse). This technique suffers from a drawback in that it imposes the use of spin echo pulses. This has the disadvantage in the first place of multiplying the duration of NMR sequences by two and, in the second place, of prohibiting the use of special sequences of the SSFP type, for example, in which it is sought to achieve dynamic equilibrium of magnetization of particles during a series of sequences in order to have a sufficient detectable signal level. Another technique is not concerned with the method of excitation but is rather concerned with fabrication technology. In this technique, inhomogeneity correction coils or so-called shim coils are associated with the magnets and with the coils for production of the orienting field. The practical application of these coils is highly complex. Inhomogeneities of the order of one millionth can be obtained only with great difficulty. Existing NMR machines are therefore difficult to construct, to adjust and to use.
The present invention has for its object to overcome these disadvantages by proposing a different generation of NMR machine which essentially consists in dissociating the polarizing and resonance effects of the orienting field. The main principles of a machine in accordance with the invention consist in subjecting the particles to a high polarizing field of 1 Tesla, for example, whereupon this field is cancelled. The magnetization conferred on the magnetic moments will therefore tend to be damped. But cancellation of the polarizing field will be more rapid than decay of magnetization of the body particles. During decay of said magnetization of the body particles, the effects of another field are employed, namely a so-called resonance field which can be of much lower strength but on the other hand much more homogeneous. By virtue of the fact that the polarizing field is of high strength, the utilizable magnetization is of high value in spite of its decay. The detectable signal is therefore strong.
On the other hand, by reason of the fact that the resonance field is low, the resonance frequency of the NMR signal associated with the excitations is low and the noise is also low. Moreover, since the frequency of the resonance signal is low, the influence of inhomogeneities is proportionally much smaller in respect of experiment times of the same order as before. In fact, if a resonance field of 100 Gauss is chosen instead of one Tesla, the resonance frequency will be divided by 100 and the time interval after which the signals from regions of the space in which inhomogeneities of one millionth prevail will now oppose their contributions only after a time interval which is one hundred times longer. The same signal-to-noise ratio for a given polarization is therefore retained but an advantage is also offered in regard to the need to reduce the inhomogeneities of the resonance field. Moreover, a high inhomogeneity of the polarizing field can be tolerated even if it has a value of the order of 3 dB, for example, since it only has the effect of unobjectionable weighting of the detected signal.